1. Field of the Invention
The present invention relates to an improved binder applied to a toner for electrophotography. More particularly, the present invention relates to a resin composition of a binder for electrophotographic toners which is capable of being used with a heat-roll fixing method.
2. Description of the Prior Art
In electrophotography, the heat-roll fixing method has been widely used to fix electrostatic latent images to be visualized with a developing toner. The toner used for this method is required to have the following characteristics: The minimum fixing temperature (abbreviated MF hereinafter) is low. Secondly, the image-offsetting temperature of the heat-roll is high. Thirdly, stability of the toner in storage is good, i.e., powders of the toner are not easily coagulated.
Japanese Pat. Publication No. 60-20411 and Japanese Pat. Tokkai Sho No. 61-215558 disclose an invention wherein a binder that possesses a wide range of molecular weight distribution over from a low molecular weight to a high molecular weight, and whose glass transition temperature is 5.degree. to 80.degree. C., is applied to a toner to meet the above mentioned three requirements. Also Japanese Pat. Tokkai Sho No. 60-45259 discloses a toner binder wherein a copolymer of a styrene series monomer and (meth)acrylic ester (hereinafter, the meaning of (meth)acryl- is not specified as either acryl- or methacryl-) is divided into three fractions at the boundaries of molecular weight 100,000 and 200,000 by employing gel permeation chromatography, and a certain amount of the obtained component possessing a glass transition temperature in a certain range is mixed, respectively. Further U.S. Pat. No. 4,499,168 discloses a toner binder consisting of a styrene-(meth)acrylate copolymer (A) having its molecular peak at 5,000 to 80,000, and a styrene-(meth)acrylate copolymer (B) having its molecular peak at 100,000 to 2,000,000 when both of these copolymers being are fractionated by gel permeation chromatography. The mixing proportion of copolymers A:B is in a weight ratio of 2:1 to 1:50.
However, with a progress of high-speed copying by electrophotography, a toner in which the MF value is lower than that of conventional toners besides the HO value is not lower has been recently required. However, toners comprising conventional binders possess the drawback that a lower MF value causes lower a HO value and poor stability in storage, and conversely, a higher HO value and good stability in storage cause the MF value to be higher. Japanese Pat. Tokkai Sho No. 60-45259, for example, discloses a toner binder consisting of 20 to 30 weight % of styrene-(meth)acrylate copolymers having a glass transition temperature between 35 and 50.degree. C.; 5 to 15 weight % of styrene-(meth)acrylate copolymers having a glass transition temperature between 35.degree. and 60.degree. C.; and 60 to 70 weight % of styrene-(meth)acrylate copolymers having a glass transition temperature between 50.degree. and 100.degree. C. Since the toner employing this binder has a low MF value but possesses a high HO value, and poor stability upon storage, it is difficult to employ this toner for recent high-speed copying purposes.
Therefore, an object of the present invention is to provide a toner binder for electrophotography which may be followed employed in high-speed of of copying by electrophotography.
In other words, an object of the present invention is to provide a binder capable of providing a toner possessing a low MF value, a high HO value, and good stability upon storage which is used for electrophotography.